1. Field of the Invention
The present invention relates to a method of fabricating an inner roller or an outer roller in an internal-meshing planetary gear construction.
2. Description of the Prior Art
There has heretofore been extensively known an internal-meshing planetary gear construction having a first shaft, eccentric elements which are rotated by the rotation of the first shaft, external gears which are assembled so as to be respectively rotatable eccentrically to the first shaft through the corresponding eccentric elements, an internal gear with which the external gears are "in internal mesh" (that is, with which the external gears mesh internally of this internal gear), and a second shaft which is connected to the external gears through a device for transmitting only the rotational components of the external gears on the axes thereof.
An example of the construction in the prior art is illustrated in FIGS. 3 and 4. The prior-art example is such that the construction is applied to "reduction gears" by employing the first shaft as an input shaft and the second shaft as an output shaft and by fixing the internal gear.
Eccentric elements 3a, 3b are snugly fitted on the input shaft 1 with a predetermined phase difference (180.degree. in this example) set therebetween. The eccentric elements 3a and 3b are integral in this example. Two external gears 5a, 5b are mounted on the respective eccentric elements 3a, 3b through corresponding bearings 4a, 4b. Each of the external gears 5a and 5b is formed with a plurality of inner roller holes 6 (6a and 6b), in each of which an inner pin 7 and an inner roller 8 are inserted.
The inner pin 7 is enclosed or concealed with the inner roller 8 for the purpose of dispersing slips during the operation of the planetary gear construction, that is, the slips between the inner pins 7 and the external gears 5a, 5b are dispersed into the slips between the inner pins 7 and the inner rollers 8, and the slips between the inner rollers 8 and the external gears 5a, 5b.
The inner pins 7 and inner rollers 8 which penetrate through the external gears 5a, 5b are secured or fitted in the flange portion of the output shaft 2.
The 2 (two) external gears 5a, 5b (in a double-row structure) are chiefly intended to enlarge a transmission capacity, to maintain a strength and to hold a dynamic rotational balance.
External teeth 9 of trochoidal tooth profile, circular-arc tooth profile, or the like are provided at the outer periphery of each of the external gears 5a, 5b. The external teeth 9 are in internal mesh with the internal gear 10 fixed to a casing 12.
The internal teeth of the internal gear 10 are concretely constructed of outer pins 11. The outer pins 11 are loosely fitted in outer pin holes 13, and are held easy of rotation. Each of the outer pins 11 is sometimes enclosed with an outer roller 14 as shown in FIG. 5 by way of example. Thus, slips during the operation can be dispersed (that is, the slips between the outer pins 11 and the outer pin holes 13 in FIG. 4 can be dispersed into the slips between the outer pins 11A and the outer rollers 14, and the slips between the outer rollers 14 and the outer pin holes 13 as understood from FIG. 5).
The operation of the exemplified reduction gears will be briefly explained. When the input shaft 1 is rotated one revolution, the eccentric elements 3a, 3b is also rotated one revolution. When the eccentric elements 3a, 3b perform one revolution, the respective external gears 5a, 5b are about to rock (or swing) and rotate around the input shaft 1. Since, however, the rotations of the external gears 5a, 5b on the axes thereof are restrained by the internal gear 10, the external gears 5a, 5b perform almost only the rocking in internal mesh with the internal gear 10.
Now, supposing by way of example a case where the number of teeth of each of the external gears 5a and 5b is N and where the number of teeth of the internal gear 10 is (N+1), the difference between the numbers of teeth is 1 (one). Consequently, each time the input shaft 1 is rotated one revolution, the external gears 5a and 5b shift (or rotate on their own axes) to the amount of one tooth relative to the internal gear 10 fixed to the casing 12. This signifies that speed of one rotation of input shaft 1 is reduced to speed of -1/N rotation of the external gears 5a, 5b. Here, the minus sign indicates the reverse rotation (or the revolution in the reverse direction).
The rotations of the external gears 5a, 5b have the rocking components thereof absorbed by clearances defined between the inner roller holes 6 and the inner rollers 8, and only the rotational components thereof on their own axes are transmitted to the output shaft 2 through the inner pins 7 inserted in the inner rollers 8. As a result, speed reduction at a reduction ratio of -N is eventually accomplished.
By way of example, accordingly, a geared motor of great reduction ratio can be obtained with only one stage of reduction mechanism by combining the reduction gears of the internal-meshing planetary gear construction with an electric motor.
In the prior-art example, the internal gear of the internal-meshing planetary gear construction is fixed, and the first shaft and second shaft are respectively employed as the input shaft and output shaft. However, reduction gears can also be constructed by fixing the second shaft, employing the first shaft as an input shaft and employing the internal gear as an output shaft. Further, speedup gears can also be constructed by inverting the input/output relations of each of such reduction gears.
Besides, in the prior-art example, the eccentric elements are directly mounted on the outer periphery of the first shaft. In this regard, there has also been known a construction of the type wherein the first shaft is dispersed or divided into "three first shafts" through spur gears, eccentric elements are respectively mounted on the dispersed first shafts, and the external gears are rockingly rotated through the eccentric elements. The present invention is applicable even to the internal-meshing planetary gear construction of such a type without any problem.
Meanwhile, as exaggeratedly shown in FIG. 6, a clearance .delta.1 is defined between the outer periphery of the inner pin 7 and the inner periphery of the inner roller 8. Besides, as exaggeratedly shown in FIG. 7, a clearance .delta.2 is defined between the outer periphery of the outer pin 11A and the inner periphery of the outer roller 14. Each of the clearances .delta.1 and .delta.2 serves to ensure the formation of a lubricating oil film between the two members, and to allow the smooth slip between the members touching with each other.
However, when such a clearance .delta.1 or .delta.2 is defined, the problem arises that backlash develops between the inner pin 7 and the inner roller 8 or between the outer pin 11A and the outer roller 14, with the result that the whole gear transmission mechanism undergoes backlash. This incurs the disadvantage that, to when the rotation on one side is to be transferred to the rotation on the other side, the rotation on the driving side does not immediately appear as the rotation on the driven side. Hereinbelow, such a delay in response shall be termed "angular backlash".
In a case where the internal-meshing planetary gear construction is used as the control mechanism of, for example, a servomotor, the angular backlash degrades the control precision thereof. Various causes are considered for the occurrence of the angular backlash in the internal-meshing planetary gear construction. As contrivances for eliminating such angular backlash, there have hitherto been known various structures, for example, one wherein the external gears, the internal gear, etc. are bisected for forward rotation and for reverse rotation, and one wherein the roles of the external gears, the internal gear, etc. are allotted to the forward rotation and to the reverse rotation. In these techniques, for example, the external gears for forward rotation are mounted with no angular backlash against the direction of forward rotation, and used only for forward rotation. On the other hand, the external gears for reverse rotation are also mounted with no angular backlash against the direction of reverse rotation and used only used for reverse rotation.
In addition, the inventors have even proposed an expedient as a method of minimizing a clearance relevant to an outer pin and an outer pin hole (in an internal-meshing planetary gear construction of the type which has no outer roller). In this technique, the outer pin and outer pinhole are contacted with each other only at 3 points, so that two oil pockets therebetween are formed to prevent seizure thereof.
In any of the known examples, however, note has never been taken of the clearance .delta.1 between the inner pin and the inner roller or the clearance .delta.2 between the outer pin and the outer roller for the purpose of relieving the angular backlash. It has been the actual circumstances that quite no measure is taken to counter the angular backlash developing in the clearance .delta.1 or .delta.2.
The reason therefor is that the clearance .delta.1 or .delta.2 has been considered unremovable (as an indispensable constituent) because of the following requisites: 1 Predetermined lubricating oil film needs to be always formed between the inner pin and the inner roller or between the outer pin and the outer roller. 2 Even at the appearance of the state in which the axes of the inner pin and the inner roller or those of the outer pin and the outer roller have deviated due to a machining error, an assemblage error, or the deformation of the two members during the transmission of power, both the members need to be slipped smoothly.
Incidentally, the use of a material of low friction and good affinity, for example, white metal or fluorocarbon resin is also considered as a method which ensures the favorable slides of the two slide members without defining the clearance. In general, however, a great torque with the torque of the input shaft amplified several times to 100 or more times, acts on the inner roller or outer roller of the internal-meshing planetary gear construction. Therefore, a material of high hardness and high strength must be used from the viewpoint of durability, and the above method cannot be adopted in many cases.
Further, in this regard, the material of high hardness and high strength needs to be wrought at a high precision, so that the inside and outside diameters of the inner roller or the outer roller must be finished by "grinding". Especially in case of finishing the inside diameter by the grinding, a finish roughness is limited (to 2-3 .mu.m! as an economical value) for the reason that the grinding is, to the last, a work for shearing the crystal grains of the material. This has led to the requisite that the existence of the clearance .delta.1 or .delta.2 having a certain magnitude is indispensable for keeping an oil film under the condition of the roughness.
For the reasons thus far explained, the clearance .delta.1 between the inner pin and the inner roller or the clearance .delta.2 between the outer pin and the outer roller has hitherto been thought indispensable. Accordingly, the occurrence of the angular backlash ascribable to the clearance has been thought inevitable.